“Quiescing” is a term used to describe pausing or altering a state of processes executing on a computer, particularly processes that might modify information stored on a storage device during a backup, thereby guaranteeing a consistent and usable backup. Quiescing typically requires flushing any outstanding writes to the storage device.
In computer storage systems, application consistent snapshots comprise snapshots taken while an application is quiesced. Application consistent snapshots typically require special handling during both snapshot creation and snapshot restoration in order to maintain consistency. Part of this operation is to ensure that all application data is quiesced in the storage medium before the backup/snapshot occurs.
A snapshot comprises a copy of data residing on a storage volume (e.g., a disk drive) that is created at a particular point in time. Since a full backup of a large data set can take a long time to complete, a snapshot may define the dataset to be backed up. Data associated with the snapshot is static, and is therefore protected from any subsequent changes to the data on the volume (e.g., a database update).
One typical implementation of a snapshot is called a “pointer snapshot.” A pointer snapshot records an index of data locations to be protected on the volume. Pointer snapshots can be created fairly quickly and require far less storage space than is required to maintain a separate copy of the snapshot data.